The present invention is directed to fluorosilicone cross-linkers and their use in room temperature condensation curable organo(fluoro)polysiloxane compositions useful in the preparation of solvent resistant sealants.
Since the early 1950""s when integral fuel tanks became a common structural feature of aircraft, fuel resistant sealing materials, such as polysulfide polymers, were used to contain the fuel. Initially, polysulfide polymers were employed because of their excellent fuel resistance. In addition to fuel resistance, elongation, flexibility, and tensile strength are additional properties needed in a fuel tank sealant. For example, upon cure, a typical fuel tank sealant may show at room temperature, typical values in a range between about 250 and about 300 elongation (%), a tensile strength (psi) of about 150, and a 100% modulus (psi) in a range between about 50 and about 100. An evaluation of available fuel resistant sealants showed that fluorinated organopolysiloxane polymers possess many desirable properties. However, commercially available fluorosilicone sealants are often based on a one-package moisture curable acetoxy cure system which results in the generation of corrosive volatiles, such as acetic acid.
A fluorosilicone composition having a neutral condensation cure system is shown by Fujiki, U.S. Pat. No. 5,236,997, who employs a fluorine containing polydiorganosiloxane base polymer having a reduced level of silicon bonded, fluorine containing substituent groups in the terminal position. Fujiki resorts to the synthesis of a special fluorosilicone base polymer substantially free of bulky fluorine containing organic groups in the terminal positions. The resulting chain-end modification to make curable polymers requires an additional step in the process. In addition, any reduction in the fluorine level in the base polymer typically results in a decrease in fuel resistance performance.
With the need for fuel tank sealants which do not corrode, neutral condensation cure systems for commercially available silanol terminated fluorine containing polydiorganosiloxane base polymers which do not require any alteration in the fluorine content are constantly being sought.
The present invention provides a neutral, room temperature condensation curable, fluorosilicone sealant composition comprising:
(A) a silanol terminated fluoroalkyl substituted polydiorganosiloxane,
(B) an oligomeric fluorosilicone crosslinker,
(C) filler, and
(D) an effective amount of a condensation catalyst,
A further embodiment of the present invention provides an oligomeric fluorosilicone crosslinker of the formula,
(R2O)m(R)nSiOxe2x80x94[R(R1)SiO]xxe2x80x94Si(R)n(OR2)m,xe2x80x83xe2x80x83(2)
where R is a C(1-12) organo radical, R1 is a C(3-8) fluoroalkyl radical, R2 is a C(1-12) alkyl radical, xe2x80x9cmxe2x80x9d is an integer having a value of 2 or 3, xe2x80x9cnxe2x80x9d is an integer having a value of 0 or 1, and the sum of xe2x80x9cm+nxe2x80x9d is equal to 3, and xe2x80x9cxxe2x80x9d is an integer in a range between about 3 and about 30 inclusive.
A further embodiment of the present invention provides a method for making an oligomeric fluorosilicone crosslinker having terminal polyalkoxysiloxy units which comprises effecting reaction between a silanol terminated polyfluoroalkylsilicone fluid and a polyalkoxysilane in the presence of an effective amount of a Platinum Group Metal catalyst.
In preparing the neutral, room temperature condensation curable, solvent resistant sealant compositions of the present invention, it is preferred to initially prepare a xe2x80x9cpastexe2x80x9d in the form of a substantially uniform blend of a filler and a silanol terminated fluoroalkyl substituted polydiorganosiloxane. xe2x80x9cNeutralxe2x80x9d as used herein refers to a sealant composition which is substantially acid-free and substantially base-free. Additional components, such as a heat stabilizer can be added. Any number of methods for blending said components known in the art may be utilized, such as batchwise shearing in a double planetary, change-can type mixer. Preferably, the paste is prepared in a continuous fashion on a devolatilizing, counter-rotating, non-intermeshing twin screw extruder, as taught in U.S. Pat. No. 4,528,324, U.S. Pat. No. 5,354,833, and U.S. Pat. No. 5,514,749. Blending of the ingredients is typically carried out using external heating at temperatures in a range between about 50xc2x0 C. and about 200xc2x0 C., preferably in a range between about 100xc2x0 C. and about 150xc2x0 C. A vacuum also can be used on the paste to degas, deaerate, or combinations thereof to achieve a substantially uniform blend.
Commercially available silanol terminated fluoroalkyl substituted polydiorganosiloxanes having a viscosity at about 25xc2x0 C. in a range between about 6xc3x97104 centipoise and about 1.6xc3x97105 centipoise can be used in the practice of the invention to make the neutral, room temperature condensation curable, solvent resistant sealant compositions. While the silanol terminated fluoroalkyl substituted polydiorganosiloxanes preferably include chemically combined trifluoropropylmethylsiloxy units, other alkylfluoroalkylsiloxy units also can be present, such as different C(1-12) alkyl radicals, for example, radicals, such as methyl, ethyl, propyl , and butyl and phenyl, and other C(3-8) fluoroalkyl units. The silanol terminated fluoroalkyl substituted polydiorganosiloxane comprises organofluorosiloxy units of formula (1),
R(R1)SiOxe2x80x94,xe2x80x83xe2x80x83(1)
where R is a C(1-12) organic radical, and R1 is a C (3-8) polyfluoroalkyl radical. The silanol terminated fluoroalkyl substituted polydiorganosiloxane is typically present at about 80 parts per 100 parts by weight of the total sealant composition.
Some of the condensation catalysts which can be used in the neutral condensation curable room temperature fluorosilicone sealant compositions of the present invention, include but are not limited to, dibutyltin diacetate, dimethyltin neodecanoate, dibutyltin dilaurate, stannous octoate, dimethyltin hydroxyoleate, or combinations thereof. An effective amount of the condensation catalyst is in a range between about 0.1 part and about 5.0 parts by weight per 100 parts by weight of sealant composition, and preferably in a range between about 0.1 part and about 1.0 parts by weight per 100 parts by weight of sealant composition.
While fumed silica is preferably used in the sealant composition as a reinforcing filler, extending fillers, such as diatomaceous earth, precipitated silica, ground quartz, and calcium carbonate, also can be employed in particular instances. It is preferred to use fumed silica which has been pretreated with an effective amount of a cyclic siloxane, such as octamethylcyclotetrasiloxane, or a mixture thereof with an organosilazane, such as hexamethyldisilazane. There can be used in a range between about 0 parts and about 30 parts by weight of filler per 100 parts of the silanol terminated fluoroalkyl substituted polydiorganosiloxane and preferably, in a range between about S parts and about 15 parts by weight of filler per 100 parts of the silanol terminated fluoroalkyl substituted polydiorganosiloxane. In addition to reinforcing or extending fillers, heat stabilizers such as iron oxide, ceric oxide, and titanium dioxide, also can be employed in a range between about 0.1 parts and about 10 parts by weight per 100 parts by weight of the silanol terminated fluoroalkyl substituted polydiorganosiloxane.
The neutral room temperature condensation curable fluorosilicone sealant composition is prepared by blending the oligomeric fluorosilicone cross-linkers, shown by the following formula:
(R2O)m(R)nSiOxe2x80x94[R(R1)xe2x80x94SiO]xxe2x80x94Si(R)n(OR2)mxe2x80x83xe2x80x83(2),
which have terminal polyalkoxysiloxy units in combination with the above-described paste. The oligomeric fluorosilicone cross-linker is typically present in a range between about 1 part and about 20 parts by weight per 100 parts by weight of the silanol terminated fluoroalkyl substituted polydiorganosiloxane.
As shown within formula (2), R is a C(1-12) organo radical, R1 is a C(3-8) polyfluoroalkyl radical, R2 is a C(6-12) alkyl radical, xe2x80x9cmxe2x80x9d is an integer having a value of 2 or 3, xe2x80x9cnxe2x80x9d is an integer having a value of 0 or 1, and the sum of xe2x80x9cm+nxe2x80x9d is equal to 3, and xe2x80x9cxxe2x80x9d is an integer having a value in a range between about 3 and about 30 inclusive. Radicals included within R are for example, C(1-12) alkyl radicals, such as, methyl, ethyl, propyl, butyl; C(6-12) aryl radicals and halo aryl radicals such as phenyl tolyl, xylyl, chlorophenyl, and naphthyl. Radicals included within R2 are for example methyl, ethyl, propyl and butyl. Preferably, R is methyl, R1 is trifluoropropyl, and R2 is ethyl.
The neutral, room temperature condensation curable, solvent resistant sealant composition is prepared by blending the desired crosslinker with the paste described above, within the aforedescribed proportions. An effective amount of a condensation curing catalyst is typically incorporated into the resulting blend as a separate component at the time of use to afford a room temperature vulcanizing rubber. Alternatively, both the crosslinker and condensation curing catalyst may be kept separate from the polydiorganosiloxane-filler paste until curing of the composition is desired, at which time these components, either separately or together are mixed with the paste to afford a room temperature vulcanizing rubber.
The oligomeric fluorosilicone cross-linker of formula (2), having terminal alkoxysiloxy units can be made by effecting contact in substantially anhydrous conditions at a temperature in a range between about xe2x88x9210xc2x0 C. and about 150xc2x0 C, preferably in a range between about 10xc2x0 C. and about 40xc2x0 C., between a silanol terminated fluoroalkyl substituted polydiorganosiloxane of formula (3)
HOxe2x80x94[R(R1)SiO]xxe2x80x94OH,xe2x80x83xe2x80x83(3)
having a viscosity in a range between about 40 centipoise and about 200 centipoise at about 25xc2x0 C., and a polyalkoxysilane of formula (4),
HSi(R)n(OR2)m,xe2x80x83xe2x80x83(4)
in the presence of an effective amount of a Platinum Group Metal catalyst, or xe2x80x9cPGM catalystxe2x80x9d where x, n, m, R, R1, and R2 are as previously defined. Reaction is typically carried out until cessation of gas evolution. Preferably, the polydiorganosiloxane includes chemically combined [trifluoropropyl(methyl)silyloxy] units.
Among the polyalkoxysilanes of formula (4), there are preferably included triethoxysilane, trimethoxysilane, methyldiethoxysilane, methyldimethoxysilane, or combinations thereof.
While the PGM catalyst used in the practice of the invention to synthesize the cross-linker is preferably platinum, the PGM catalyst also can include compounds of ruthenium, osmium, iridium, palladium, cobalt, rhodium and nickel. In forming the cross-linker, an effective amount of the PGM catalyst is in a range between about 10 parts per million (ppm) and about 103 parts per million of PGM, per part of reaction mixture.
Among the PGM catalysts which can be used, include a platinum catalyst as shown by Karstedt, U.S. Pat. No. 3,775,452, which is formed by reacting chloroplatinic acid with tetramethyldivinyldisiloxane in the presence of sodium bicarbonate in ethanol. Further examples are shown by Lamoreaux, U.S. Pat. No. 3,220,972 directed to reaction products of chloroplatinic acid and alcohols, ethers, aldehydes, and mixtures thereof and reaction products of an olefin and chloroplatinic acid, as shown in Ashby, U.S. Pat. No. 3,159,601, or the reaction product of platinic chloride and cyclopropane as described in Ashby, U.S. Pat. No. 3,159,662. Preferably, the PGM catalyst is a heterogeneous platinum/carbon catalyst or palladium/carbon catalyst.
The crosslinker in the present invention can be employed in non-corrosive cure systems to fabricate elastomers useful as fuel resistant sealing materials, electronic encapsulation, and in applications requiring chemically resistant materials. Particularly, the crosslinkers can be used for preparation of fuel tank sealants.